SMD (Surface Mount Device), such as passive components selected from resistors, inductors or capacitors, are mounted on the contact pads of a circuitized substrate like printed circuit board or ceramic substrate. Recently, more and more components and SMD's are requested to mount on a chip-carrying substrate especially for Multi-Chip Package/Module, Chip Scale Package, or System-In-Package. However, the substrate has a limited area for SMD's. Generally, solder paste serves as an electrical connection agent for mounting SMD on the contact pads of a substrate. The solder paste will be pressed by the SMD and caused uncontrollably flow of solder paste on the contact pads during reflowing. After reflowing, the remains of solder paste can be easily condensed and formed solder beads due to the large surface tension of solder. The solder beads will be free to move about on the substrate, resulting in severe contamination, moreover, electrical problems.
Referring to FIG. 1, a conventional circuitized substrate 10 includes a first contact pad 20 and a second contact pad 30 for mounting a SMD 40. Normally the contact pads 20 and 30 are square and close to each other so as to connect with the first terminal 41 and the second terminal 42 of the SMD 40 respectively. Referring to FIG. 2 and 3, the first contact pad 20 and the second contact pad 30 are exposed out of solder mask openings 12. Solder paste 50 is applied on the first contact pad 20 and the second contact pad 30. When the SMD 40 is mounted on the circuitized substrate 10, solder paste 50 is reflowed to assure good electrical connection between the first terminal 41 and the first contact pad 20 and between the second terminal 42 and the second contact pad 30. The remains of solder pastes 50 under a pressure may be extruded out of the SMD 40 toward the corners of the first contact pad 20 and the second contact pad 30. Therefore, tiny solder beads 51 can easily form after reflowing the solder paste 50. The solder beads 51 are formed at the right-angled or acute-angled corners 21, 31 or randomly on the solder mask, and can easily move about on the substrate 10 due to poor adhesion of solder beads 51 to the solder mask which will cause electrical problems.
A conventional contact pad design for passive components is disclosed in R.O.C. Taiwan Patent publication No. 472,367. The solder mask has a radial-shaped opening and a plurality of grooves extending outward on a contact pad for connecting a SMD. The fin-shaped grooves are used to disperse solder paste to avoid formation of solder beads. However, the solder beads are formed unpredictably due to the variation of the mounting pressure of SMD and the quantity of solder paste applied. Once the solder beads are formed in the above-mentioned fin-shaped grooves, the solder beads cannot be easily bonded to the slender and acute-angled grooves which leads to short circuit. When the substrate is heated during the following processes such as high temperature molding for semiconductor packages, the solder beads will be melted and flowed back to the center of the contact pad. The SMD will become floating and can be easily swept by a molding flow. Thus, it is not good to manufacture a semiconductor package using such a substrate as a chip carrier. Besides, the solder mask with radial opening and metal contact pads are difficult to manufacture.